1. Field of the Invention
The present invention relates to a method of coating a flexible belt-shaped supporting element (hereinafter referred to as "a web" when applicable) which is continuously transported with suitable coating liquids in the manufacture of photographic films, photographic paper, instant film units, magnetic tapes, adhesive tapes, pressure-sensitive recording sheets, offset plates or the like. More particularly, the invention relates to a method for preventing a web from being heavily coated with a coating liquid at the coating start position on the web.
Such a web may be made of polyesters such as polyethylene terephthalate, polyethylene-2 and 6-naphthalate; polyolefins such a polypropylene; cellulose derivatives such as cellulosetriacetate and cellulosediacetate; plastic films of polycarbonate or the like; metallic sheets of Cu, Al, Zn, etc.; glass, porcelain and ceramic; or baryta paper and synthetic paper. According to the particular application of the method, a coating-liquid adhering base layer or back layer is provided on the web in advance. The total thickness of the web including such a layer is commonly of the order of 2 to 300.mu..
2. Description of the Prior Art
With reference to FIG. 1, in a conventional method of coating a web with a coating liquid or liquids, when the coating liquid 2 begins to contact the surface of the web 1 which is continuously moving in the direction of the arrow X in FIG. 1, the line connecting the initial contact points of the coating liquid 2 on the web 1 (hereinafter referred to as "a coating start position" when applicable) in general has a V-shaped, W-shaped or linear pattern, corresponding to FIGS. 1A, 1B and 1C, respectively, across the web 1 depending on the properties of the coating liquid, the type of the coating apparatus and the operating conditions of the coating apparatus. Furthermore, a heavy or thick coating portion 3 in which the amount of coating liquid increases over the desired value is formed across the web 1 by the coating liquid. The thickness of the heavy coating portion 3 may exceed by 50 to 500% the intended or regular thickness of a layer of coating liquid on the web. After the heavy coating portion 3 has terminated, a layer of coating liquid having the intended thickness is formed on the web as shown in FIG. 2.
A method for adequately preventing the formation of such a heavy coating portion 3 has not as yet been disclosed in the art. Heretofore a temporary countermeasure was employed in which the operating conditions of the apparatus were changed upon commencing the coating operation with the coating liquid 2 to reduce the drying load of the heavy coating portion 3 or to provide a sufficiently long drying time for the heavy coating portion thereby to prevent problems such as the wet film of coating liquid 2 sticking to the conveying guide means thereby making the film surface rough. To accomplish this, the following steps were performed.
1. The amount of coating liquid 2 supplied to the coating means was reduced to decrease the amount of coating liquid in the heavy coating portion 3.
2. The coating operation was started with the running speed of the web 1 set to a very low value after which the running speed was gradually increased to a desired value.
3. Operating conditions such as temperature, the volume of the air, and the velocity of the air in the drying device were changed for a certain period of time so that the heavy coating portion 3 could be completely dried and delivered from the drying zone.
Unfortunately, such methods are disadvantageous in that a coated film having a desired thickness and quality cannot be obtained without wasting a large amount of coating liquid.
A coating method eliminating this difficulty has been described in the specification of Japanese Laid-Open Patent No. 92328/1975. According to this method, a portion of the surface of the web 1 where the coating is to be started is roughened with a knurling tool or the like thereby to prevent the formation of the heavy coating portion.
According to this method, the heavy coating portion can be limited to about 11%. Thus, a satisfactory result can be obtained by increasing the drying capacity of the drying device by about 11%. Moreover, the number of defective articles can be greatly reduced.
However, the method is still advantageous in the following points. In using a web 1 whose surface has been roughened off the line, if the coating operation is suspended for some reasons and started again it is usually impossible to start the coating operation beginning with a particular roughened region of the web. Therefore, usually, it is necessary that a new web roll be loaded in the coating apparatus to start the coating operation beginning with a new roughened region of the new web.
If a roughened region is formed on the surface of the web on the line, then the above-described difficulty caused when the roughened region is formed off the line may be eliminated and, accordingly, the coating operation can be started beginning with any region of the web. However, in this case, it is necessary to deliver the web to the coating means without carrying along the fine dust or particles which were created while forming the roughened region. Otherwise the quality of the coated film surface would be considerably lowered by the particles and, accordingly, the number of defective articles would be increased. Therefore, it is necessary to set the running speed of the web 1 to a very low value to remove the particles from the roughened region of the web or a high performance dust remover must additionally be provided.